1. Field of the Invention
The present invention relates to an antenna installed in or on a window glass of an automobile for receiving a radio frequency signal from a portable radio transmitter for controlling the operation of an automotive equipment such as a keyless entry system for automatically unlocking a vehicle door, a similar control system for a luggage compartment lid, a control system for controlling turning on and off of a passenger compartment lamp, etc.
2. Disclosure Information
A keyless entry system for automatically unlocking a vehicle door by receiving a radio frequency signal of a frequency around 50 MHz, 100 MHz, 250 MHz or 300 MHz emitted from a radio transmitter has lately come to attract considerable attention.
In a keyless entry system disclosed in Japanese Provisional Patent Publication No. 62-37475, a pick-up consisting of a loop coil built in a vehicle pillar is used as an antenna. A problem of such a prior art antenna is that it is largely influenced by noise. Another problem is that the antenna is embedded in a vehicle body so that its tuning cannot be attained with ease.
A glass antenna consisting of conductive strips formed on a lower marginal portion of a side window glass by screen printing has been proposed for use in such a keyless entry system as disclosed in Japanese Provisional Publication No. 63-43403. A problem of this glass antenna is that opening and closing of the window causes variations of its gain so that during opening of the window the gains in some directions become so small as to cause malfunction of the keyless entry system.